Document that ``output`` parameter of configure_file is relative to `meson.current_build_dir()`

[moi15moi]

Currently, the documentation of the output doesn't mention it is relative to meson.current_build_dir():

The output file name. (since 0.41.0) may contain @PLAINNAME@ or @BASENAME@ substitutions, as well as (since 1.5.0) their indexed versions, like @PLAINNAME0@ or @BASENAME0@. In configuration mode, the permissions of the input file (if it is specified) are copied to the output file.

I believe it might be a good idea to mention it.